1. Field of the Invention
The present invention relates generally to electromagnetic interference (EMI) shielding and, more particularly, to EMI shielding that includes an expanding EMI gasket.
2. Related Art
Operation of electronic devices, such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like, generates undesirable electromagnetic interference (EMI) radiation. This EMI radiation often develops as a field or as transients within a radio frequency band, generally between about 10 KHz and 10 GHz, of the electromagnetic spectrum. EMI radiation can cause problems, because EMI radiation can interfere with proper operation of other electronic devices.
EMI shielding is commonly used to control EMI radiation. EMI shielding is typically constructed of an electrically-conductive material and is configured to absorb EMI radiation and allow the absorbed EMI radiation to flow to an electrical ground via a low-impedance radio-frequency (RF) path. EMI shielding may be employed to confine EMI radiation generated by a device or to protect the device from EMI radiation generated by other devices. Commonly, EMI shielding is provided as a housing that encloses a device or as a barrier that is inserted between pairs of devices.
To date, EMI control has proven difficult due to the ability of EMI radiation to travel through small gaps in EMI shielding. Despite being housed in EMI shielded device housings, electronic devices often receive or generate EMI radiation that enters or escapes through gaps in the device housings. For example, these device housings often have access ports, lids or side panels that provide access to devices enclosed by the device housings. Imperfect fit of these access ports, etc. can leave gaps large enough to pass significant amounts of EMI radiation. These gaps are especially common in so-called “blade systems,”which permit replaceable printed circuit boards (blades) to be plugged into the chassis through openings in front panels of the chassis.
To prevent, or at least minimize, EMI radiation from escaping or entering a blade system, each blade typically includes a handle, filler panel, bulkhead or other structure (hereinafter collectively referred to as a “bulkhead”) that includes an EMI shield. The bulkhead is configured and sized such that, when the blade is installed in a chassis, the bulkhead fills the opening, through which the blade was installed. Surfaces, edges, flanges or other structures (hereinafter collectively referred to as “surfaces” or “mating surfaces”) on the perimeter of the bulkhead are generally configured to mate with corresponding surfaces around the perimeter of the opening when the blade is installed in the chassis. Unfortunately, even strict manufacturing tolerances have proven insufficient to adequately prevent EMI radiation from passing through gaps between these mating surfaces. Therefore, a resilient, electrically-conductive EMI gasket is often attached to the perimeter surfaces of either the bulkhead or the structures that define the opening. Thus, when the blade is installed, the EMI gasket is compressed between mating surfaces of the bulkhead and the opening. (Hereinafter, a “surface” or “mating surface” also includes an EMI gasket, if one is present at a point or line of contact between a bulkhead or opening and another object.)
The compressive force between these mating surfaces is intended to urge the EMI gasket to fill any gaps, and thus maintain a continuous EMI shield, between these mating surfaces. However, large forces are required to simultaneously plug the blade into a socket of the chassis and compress the EMI gasket sufficiently to fill the gaps. These forces can unduly stress the blade or damage components or printed circuit wiring on the blade. These forces can also distort the bulkhead or opening and, thereby, cause other gaps to occur between the mating surfaces or cause existing gaps between the mating surfaces to enlarge. Furthermore, an EMI gasket's effectiveness depends on the gasket's resilience. However, the large forces necessary to compress an EMI gasket often permanently deform it or reduce its resilience, thereby making the gasket less effective or ineffective for reuse.
Some blade system chassis do not have discrete openings in their front panels for each blade. Instead, each front panel includes one large opening, through which a plurality of blades can be installed adjacent one another. In this type of blade system, an EMI gasket on each blade is intended to contact a mating surface of the adjacent blade. However, some permanently deformed, i.e. undersized, EMI gaskets or an accumulation of dimensional errors in the plurality of adjacent bulkheads can lead to an overly tight or overly loose fit among the bulkheads in a fully-populated chassis. An overly loose fit leads to gaps in the EMI shielding, whereas an overly tight fit makes it difficult or impossible to install the final blade without excessive insertion force.
There is, therefore, a need for an effective EMI sealing mechanism that can be used in blade systems without the problems of the prior art.